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Transmission Line Question - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


mikerauh
Post subject: Transmission Line Question Posted: Fri Mar 07, 2008 2:46 pm

Lieutenant

Joined: Fri Mar 07, 2008 2:29 pm
Posts: 3
In his book, Prof. Guillermo Gonzalez writes:

V(x)=V+(x) + V-(x) and

I(x)=I+(x) - I-(x)

Why is there a minus sign in the second equation? The equations say the voltage at a point on the transmission line is the sum of the forward wave voltage V+(x) and the reflected wave voltage V-(x). But the current is the difference of the current of the forward wave I+(x) and the current of the reflected wave I-(x). It seems to me the voltage and the current are vector quantities, i.e. they have a phase. So why the minus sign? Why not just sum them?

Mike Rauh


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fred47
Post subject: Posted: Fri Mar 07, 2008 4:30 pm

General


Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104
If I understand it correctly, it's to make the direction of the power flow work out correctly. Power doesn't really have a phase, but power flow definitely has a direction, and I think the signs are set up to give that.

Hope this helps!
Fred


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mikerauh
Post subject: Posted: Fri Mar 07, 2008 5:18 pm

Lieutenant

Joined: Fri Mar 07, 2008 2:29 pm
Posts: 3
Fred,

You are correct. The apparent current has to be defined as the incident wave current minus the reflected wave current to get the power relationships right.

Thank you.

Mike







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